Conventionally, in a standard computer, the superposing and synthesizing processing of the display data in a single frame memory is performed directly by a main processor or portrayal device on the memory. FIG. 1 is a block diagram showing one embodiment of the conventional image display device. This image display device is composed of a main CPU 101, a main memory 102, a data processing circuit 103, a line memory 104, an output processing circuit 105, a system controller 106, and a sync signal generating circuit 107.
In the main memory 102, some display data are stored. For example, considering a case where several kinds of window display are performed, display data corresponding to each window is stored. When these windows are superposed and displayed on one screen, the main CPU 101 selects and reads each display data so as to obtain one screen-display, and stores again the display data for one screen in the main memory 102. The system controller 106 generates an address of the main memory 102 for the data transfer, according to the timing of the sync signal generated by the sync signal generating circuit 107. After the display data is read out from the main memory 102 according to this address, and a predetermined data processing is performed by the data processing circuit 103, the data is transferred to the line memory 104. The data from the line memory 104 is output according to the timing of the sync signal, subjected to the processing for display by means of the output processing circuit 105 and displayed on the display.
Furthermore, as disclosed in Japanese Patent Application Laid-Open Hei 6 No. 149527, there is a system in which frame memories are prepared for the numbers necessary for superposition, the data is read out from all the frame memories at the time of outputting the picture, and the synthesized results are displayed based on the priority among respective frames.
Moreover, as disclosed in Japanese Patent Application Laid-Open Hei 6 No. 295169, there is a system which identifies which mode (for example, the bit number of one pixel) each display dot is in by providing an identification memory for every display dot of the memory in the display area separately from the memory for display, and displays the display dot according to the mode, and displays a different display mode on one screen.
In addition, there is a system which references the content of the identification memory, or a system which utilizes a separate mask memory and when the information in each window now being displayed is changed and rewritten, masks the outside of the area, as disclosed in Japanese Patent Application Laid-Open Hei 7 No. 334342.
When the main CPU 101 performs the processing such as superposition and the like of each window, however, the burden of the main CPU 101 becomes too much, resulting in such a problem that the main CPU 101 cannot perform other processing to decrease the overall processing speed.
Furthermore, in a method to reduce the processing load of the software by having frame memories for the numbers required to superpose each window, it is required from the initial stage to have the frame memories of the maximum numbers to be considered necessary. Namely, regardless of the size of the window to be displayed on the screen, the frame memories are required in the maximum size of the display area. Therefore, the efficiency of using the memory is decreased, and when a number of windows are open at the same time, it is required to read out the data simultaneously from the whole frame memories corresponding to the window. Namely, it is necessary to read out the data whose window is superposed and not displayed actually. Thus, the consumed power becomes large proportional to the number of windows opened on the screen.
Furthermore, as a method to mingle and display different display modes on one screen as in the conventional device, there can be mentioned a method to identify which mode each display dot is in by providing identification memories for each display dot of the memory in the display area. In this method, since the identification memory of several bits becomes necessary separately for the memory for the full screen, a memory (identification memory) becomes necessary on the side which cannot be used for other applications. Similar thing can be said when the mask memory is used.
It is an object of the present invention to provide a programmable display device which requires only a memory space for storing the display data, and which can increase the processing speed by reducing the number of access to the memory for the display and reduce the burden of the main control section.